Prevent crash if negative change in sweep

Fixes #976. Joinmarket will allow, but warn, when
non-zero change values (which are not included in
the transaction) result from sweeps (due to the
impossibility of knowing the exact value in advance).
However, prior to this commit, if that estimation
inaccuracy resulted in a negative change value that
was to be ignored, there was a crash due to the fact
that `jmbitcoin.amount_to_str` does not allow negative
values. Hence we instead print the number in the warning
without going through this formatting function.
Adds tests case for negative sweep change.

jmclient/jmclient/taker.py

@@ -475,7 +475,7 @@ class Taker(object):
                 # fees is acceptable
                 jlog.info(
                 ('WARNING CHANGE NOT BEING USED\nCHANGEVALUE = {}').format(
-                    btc.amount_to_str(my_change_value)))
+                    my_change_value))
             # we need to check whether the *achieved* txfee-rate is outside
             # the range allowed by the user in config; if not, abort the tx.
             # this is done with using the same estimate fee function and comparing

jmclient/test/test_taker.py

@@ -338,7 +338,9 @@ def test_auth_pub_not_found(setup_taker):
          2, False, None, None), #tumble style non-int amounts
         #edge case triggers that don't fail
         ([(0, 0, 4, "mxeLuX8PP7qLkcM8uarHmdZyvP1b5e1Ynf", 0, NO_ROUNDING)], False, False,
-         2, False, None, None), #sweep rounding error case
+         2, False, None, None), #sweep rounding error case 1
+        ([(0, 0, 4, "mteaYsGsLCL9a4cftZFTpGEWXNwZyDt5KS", 0, NO_ROUNDING)], False, False,
+         2, False, None, None), #sweep rounding error case 2
         ([(0, 199856001, 3, "mnsquzxrHXpFsZeL42qwbKdCP2y1esN3qw", 0, NO_ROUNDING)], False, False,
          2, False, None, None), #trigger sub dust change for taker
         #edge case triggers that do fail
@@ -448,7 +450,7 @@ def test_taker_init(setup_taker, schedule, highfee, toomuchcoins, minmakers,
     if schedule[0][3] == "mxeLuX8PP7qLkcM8uarHmdZyvP1b5e1Ynf":
         #to trigger rounding error for sweep (change non-zero),
         #modify the total_input via the values in self.input_utxos;
-        #the amount to trigger a 2 satoshi change is found by trial-error.
+        #the amount to trigger a small + satoshi change is found by trial-error.
         #TODO note this test is not adequate, because the code is not;
         #the code does not *DO* anything if a condition is unexpected.
         taker.input_utxos = copy.deepcopy(t_utxos_by_mixdepth)[0]
@@ -457,6 +459,14 @@ def test_taker_init(setup_taker, schedule, highfee, toomuchcoins, minmakers,
         res = taker.receive_utxos(maker_response)
         assert res[0]
         return clean_up()
+    if schedule[0][3] == "mteaYsGsLCL9a4cftZFTpGEWXNwZyDt5KS":
+        # as above, but small -ve change instead of +ve.
+        taker.input_utxos = copy.deepcopy(t_utxos_by_mixdepth)[0]
+        for k,v in iteritems(taker.input_utxos):
+            v["value"] = int(0.999805028 * v["value"])
+        res = taker.receive_utxos(maker_response)
+        assert res[0]
+        return clean_up()
 
     res = taker.receive_utxos(maker_response)
     if minmakers != 2: